In recent years, with the increase in interest in conservation of global environment and effective utilization of resources, recycle and reuse have been promoted in the field of industrial products. Also for flue gas denitration catalysts, various attempts at recycle and reuse have been made through, for example, a method for reusing a degraded spent catalyst after cleaning the catalyst with any chemical agents, or a method for recovering useful elements from a spent catalyst.
Catalysts composed of titanium oxide carrying active components such as vanadium (V), molybdenum (Mo), or tungsten (W) oxides have been widely used as flue gas denitration catalysts. Their degradation is caused by, for example, the deterioration of the denitration activity due to the adhesion of dust components derived from the gas, for example, alkali metal elements such as potassium and sodium, alkaline earth metal elements such as calcium, and volatile compounds such as arsenic, lead, and phosphorus, the inactivation of the denitration catalytic site due to the crystal growth of titanium dioxide (TiO2) and active components by heat, and the sulfatization of the active component upon reaction with SOx. In order to solve these degradation problems, various regeneration methods have been studied and invented.
Denitration catalysts are disabled by the above-described degradation, as well as the increase of the SO2 oxidation activity of the catalyst due to the adhesion of V and iron (Fe) compounds, which are derived from the waste gases, on the catalyst surface, or the migration of Fe ions into the catalyst caused by the corrosion of the metal substrate. When SO2 contained in waste gases is oxidized to SO3 at a high ratio, or the SO2 oxidation rate is high, the amount of an acidic ammonium sulfate deposit in the air preheater, which is located in the downstream part, increases to cause the increase of the pressure loss, or SO3 is emitted in the form of SO2 through a chimney to cause violet smoking or secondary nuisance. It is thus preferred that the catalyst be replaced upon the detection of the increased SO2 oxidation rate, before the deterioration of the denitration activity. In recent years, mainly in the United States, flue gas denitration devices have been increasingly installed in boilers for burning low rank coal containing large amounts of S and Fe components. As a result of this, more catalysts are replaced upon the increase of the SO2 oxidation rate caused by the adhesion of the Fe component. Therefore, there is a strong demand for the development of a method for regenerating spent catalysts through effective decrease of the increased SO2 oxidation rate of the catalyst.
In addition to the demand to decrease the SO2 oxidation rate of denitration catalysts, also increasingly demanded is an increase of the oxidation activity of denitration catalysts for metal mercury. In recent years, in the United States and European countries, tightened regulations are imposed on the emission of trace components contained in waste gases emitted from thermal power plants. In particular, the emission of mercury (Hg) must be limited to an extremely low level, because it adversely affects the growth of infantile brain and nerves. In order to prevent the emission of mercury, much attention is paid to a method including steps of oxidizing metal mercury having a high vapor pressure with a denitration catalyst to a mercury compound having a low vapor pressure, and then recovering the mercury compound. Under the circumstances, there is an increasing demand for an advanced catalyst regeneration technique which decreases the SO2 oxidation rate, as well as improves the Hg oxidation activity of the catalyst.
Known methods for regenerating denitration catalysts are aimed at spent denitration catalysts which have been used in denitration devices for waste gases produced by burning of high rank coal containing small amounts of Fe and S components. Therefore, many of them are intended mainly to recover the denitration capability. The main purpose of these methods is not to recover the denitration activity for decreasing the SO2 oxidation rate. Examples of known methods for regenerating spent denitration catalysts include a method including steps of immersing a spent denitration catalyst in an aqueous solution of a molybdenum compound such as molybdenum trioxide, or an aqueous solution containing an ammonium salt of oxalic acid or tungstic acid, and then drying the catalyst (Patent Documents 1 and 2).
Patent Document 1: Japanese Patent Application Laid-Open No. 2004-298760
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-267897
However, 3000 ppm or more of SO2 is contained in waste gases emitted from boilers burning high-S coal such as eastern coal in the U.S.A., so that more than 30 ppm of SO3 is emitted even though the SO2 oxidation rate is 1%. Regeneration of a catalyst used in such a boiler requires effective decrease of the increased SO2 oxidation rate to an extremely low level. Therefore, the recovery of the denitration activity is relegated to the second place, or slight deterioration of the denitration activity may be accepted.